Ordered on Friday and received on...Monday ! in a tiny discrete package. So very fast "logistic" even if Lyon is not so far
So, what can I say ? Many comments have already been spraid.On my side, pros are :
- Very nice shape with very nice pouch. The calc is inserted/removed without any force at the opposite of the hp42s and seems more durable
- Feels heavy and strong with metal housing
- The general black aspect (case+layer background+most of the screen saver pictures) give a global quality and serious aspect (same spirit as could be RC models, even if it seems difficult to make DM42 flying )
- Beautifull screen, no need of contrast adjusting, allowing "HiRes" graphics handled by programs and ALSO a "screensaver"
- Up to 10 lines of program display or XYZT stack + running window of original hp42 above (on top screen)
- The extra keys used directly for menus OR used as Fn functions (F1=help,...F6=Zoom In/Out)
- Extended Alpha mode by entering letters directly from each key (like on the 41) instead of using the screen menus
- Large amount of memory (6MB) for all programs' storage and more than 10 times more memory for programs (hp42 is around 6K bytes)
- Much more faster than the original (I had some moving graphics programs, no time to see the move now, when you run, it is already ended !)
- USB connection to PC to downlad/upload programs, screenshots, firmware..
- Other things certainly but I stop here

Now, some improvements that could be done (expressed as constructive remarks as this calc is almost perfect):
- Some keys labellings could be improved as x<>y, +/-, SQRT(x) with a slightly bigger font size
- Keys' surface is not flat but concave
- Keys' bounce a little bit hard (this said, we have the clic, but louder sound compared to hp)

Some ideas :
- Blue color for the "ALPHA" label above the ENTER key and blue color for all the letters/symbols placed on the keys' bottom right (so we would have a mix between 41 and 42, interesting...)
- Not sure but : add yellow labels above the top keys like "Help Menu Vol Format Zoom"
- A port expander using the mini USB port (that should be duplicated, as the IR led for the printer). To keep a slim shape, could be 2 ports allowing to use hp41 modules (yes they would need to be enhanced). Will try to make a 3D drawing

To conclude :
I am using hp calcs for 35 years (35/67/41CV/CX/15C/42S) and I can say we have here the sum of all that these different models have brought in terms of functions, ergonomy, display, memory and speed. Some cosmetics add-ons and a retro-compatibility with hp41 modules -I know it is not a DM41, but...- that I list in "Some ideas" would make it the perfect one !

Note that I put away the graphing calculators that are in a separate category, more school oriented with a lot of dedicated input windows, maybe not specifically RPN and are of a bigger size also. You can always program graph plots on the 42 by the way.

Ah! I found how to do(choose direct link as a reminder for me)
So, here it is, I made it quickly, so it is not true colors and no labels at all, just added the golden key at the end.
I have used different colors for calculator, port expander module slots and fixation clips for a better view

Finally, we get a long calculator (and you wil need another pouch) ! and I don't know if there is space enought for the inner PCB...

and my last point, more realistic, should be to have the choice to display the running window either on top (as it is) or on bottom of the display.
A quick paint arrangement to illustrate my thinkings with a running display :
It is easier to read and looks like the original display without having to look on top

It could be choosen from
Setup -> 6.Stack Layout -> 7.(_) Top run window that you can select or not

So, what do we have on 41 side, except the calculator ?
-Standard modules : most of them -except exotic ones- are already implemented (Math/Stat, 41 advantage...) or can be programmed or downloaded
-Time module is partly implemented (missing alarm/stopwatch functions only)
-Extended functions are mostly implemented also, and for the extended memory, it is already there !
-Furthermore, specific modules require an overlay that doesn't match phisically and 42 uses the custom menu
-Bar code reader is a battery sucker so certainly not suitable with CR cell voltage (or not for a long time)
-Same thing for card reader + reserved for small programs + not physically compatible

So, what's left ? and this was my idea -and also I can answer your question- : the I/O part and especially hpil module to "talk" to a voltmeter/cassette drive or printer (not IR). Retro-compatibility
Would need to implement extra functions effectively that hpil module integrates.